1. Field of the Invention
This invention relates to a method of preparing molding sand mixtures and the mixtures thereby obtained. More particularly, it relates to molding sand mixtures which harden in a short period of time, providing molds with high strength and good surface stability.
2. Description of the Prior Art
Prior art molding sand mixtures suffer from disadvantages in that their coefficients of utilization of floor area and operation efficiencies are low. This is true because when making molds of portland cement, long periods of time are consumed until the molds attain the desired compressive strength. As an earlier improved method to eliminate these disadvantages, we have developed self-hardening molding sand mixtures which comprise a mix of molding sand with a mixture of sulfate, or of sulfate and an organic carboxylic acid, with clinker powder containing calcium haloaluminate (11CaO.7Al.sub.2 O.sub.3.CaX.sub.2), or by further addition of a water reducing agent such as a .beta.-naphthalenesulfonic acid-formalin condensation product. The molds made from these mixtures are hardened rapidly; but they have a disadvantageous tendency toward insufficient stability, especially when hardened at low temperatures. This is because of the deficiency of water necessary for the cement to hydrate near the surface and of the resulting incomplete bonding among the sand grains.